TECHNICAL FIELD
[0001] The present invention relates to runflat pneumatic tires for motor vehicles and more
specifically, to bead-region chipper-type reinforcements to improve riding comfort
and tire durability at high speeds and during runflat operation.
BACKGROUND OF THE INVENTION
[0002] Modern pneumatic vehicle tires typically include a pair of axially separated, inextensible
beads which serve to hold the tire on a wheel rim as well as to provide a structural
foundation for the reinforced carcass plies to which the other tire components, such
as the sidewalls and tread, are attached. A circumferentially disposed bead filler
apex extends radially outward from each of the two beads.
[0003] One or more carcass plies extend between the two beads, by way of the sidewalls and
the tire crown. Each carcass ply has two axially opposite end portions. The end portions
of at least one carcass ply are turned up around, or clamped to, the beads, thereby
anchoring the ends of one or more carcass ply layers. During tire construction, tread
rubber and sidewall rubber are applied axially and radially outward of the one or
more reinforced carcass plies.
[0004] The bead region of the sidewall is one part of the tire that contributes a substantial
amount to the rolling resistance or rolling friction of the tire. The rolling resistance
corresponds to an energy loss taking place within the tire's structure and is related
to the cyclical flexure of the tire components, including the tread and its underlying
structures such as the belts, as well as, especially, the portions of the sidewall
that are closest to the bead regions where the flexural strain, and energy loss, is
greatest.
[0005] The energy losses associated with a tire's rolling resistance correspond to heat
accumulation within the tire's structure. Under conditions of severe operation, as
at high speed or during operation of an uninflated runflat tire having extended mobility
properties, flexure-induced heating in the bead region portion of the sidewalls can
be especially problematic.
[0006] US-A- 3,253,693 discloses data on radial and circumferential deformations within
tires. The deformations that take place in the bead region of the sidewalls result
in shearing stresses during normal operation of the tire and especially during severe
operating conditions. Bead-region shear stresses have both circumferential and radial
orientation. The resulting shear strains or deformations correlate with high flexure
within the bead regions. In the case of runflat or extended mobility technology (EMT)
tires, sidewall flexure in the bead region can lead to a shortened runflat operational
service life. More specifically, EMT tires operating under runflat conditions inevitably
undergo deterioration and failure after operation for a certain distance; often the
failure mode involves complete cracking of the parts of the tire (i.e., the chafers)
that make the most immediate contact with the steel wheel rim's radially extending
edge. The chafer cracks are oriented at 45 degrees indicating a shear strain effect
in the bead regions.
[0007] Recent investigations have shown a high difference of radial-circumferentially oriented
shear strains between the footprint area and the part of the tire that is 180 degrees
from the footprint, i.e., the top of tire. This difference between the shear strains
at the top and bottom of the tire is also referred to as the cycle amplitude of shearing
strain, a variable which, when extreme, correlates with chafer cracking during the
uninflated operation of EMT tires.
[0008] Among the methods used to reinforce the bead region of radial-ply tires is the incorporation
of "chippers." A chipper is a circumferentially deployed metal or fabric layer that
is disposed within the bead region in the portion of the tire where the bead fits
onto the wheel rim. More specifically, each of the chipper(s)(one or more) used in
each bead region of a given tire typically lies inward of the wheel rim (i.e., toward
the bead) and inward (i.e., radially inward, relative to the bead viewed in cross
section) of the portion of the ply that turns upward around the bead. Typical single
chippers are made of parallel-aligned, metal or polymer cords that are oriented at
an angle of 25 degrees with respect to the circumferential direction.
[0009] The width of the chipper is the distance to which it extends radially outward from
the bead region. The width of the chipper is one variable that can be used to "tune"
a tire's handling and steering performance. Chippers typically extend to a radial
distance of to about 20 millimeters above the wheel's rim flange.
[0010] Generally, chippers provide a stiffening influence to the radially inward portion
of the sidewall most adjacent to the bead region. The stiffening increases the resistance
to cyclical flexure of the sort referred to above. In other words, the increased stiffness
afforded by chippers works to reduce the amount of flexural deformation and resultant
shearing stresses and strains in the axially inward portions of the sidewalls that
are most immediately adjacent to the beads.
[0011] The use of wire chippers in standard non-EMT tires improves handling and steering
performance, especially at high speeds. The formation of standing waves in non-EMT
tires during high-speed operation can also be inhibited by the stiffness/damping characteristics
of the final tire design, including the choice of chipper width. Flatspotting, i.e.
the tendency of the tread of a tire to sustain a flat spot in the ground-contacting
portion of the tread when a vehicle has been parked or otherwise sitting for a prolonged
period, is also alleviated by the use of chippers.
[0012] A balanced design for a chipper-reinforced bead assembly of a tire would include
stress characteristics that lead to reduced flexural energy generation (heat buildup)
and to strain characteristics that can be uniformly borne by mutually adjacent tire
components in the bead region. The objective of a balanced design is to achieve high-speed
handling and steering benefits without compromising riding comfort due to the increased
rigidity associated with typical chipper designs.
OBJECTS OF THE INVENTION
[0013] It is an object of the present invention to provide an optimized chipper design for
use in a runflat radial tire as defined in one or more of the appended claims and,
as such, having the capability of accomplishing one or more of the following subsidiary
objects.
[0014] One object of the present invention is to provide an improved chipper design that
minimizes the shear strain cycle amplitude in the bead region of runflat tires during
runflat operation in order to minimize the formation of cracks in the chafer region.
[0015] Another object of the present invention is to provide an improved chipper design
that reduces the flatspotting tendency of a runflat tire during normal-inflated service.
[0016] Yet another object of the present invention is to provide an improved chipper design
that serves to minimize the potential for the formation of standing waves during high-speed,
normal-inflated operation.
[0017] Still another object of the present invention is to provide an improved chipper design
that contributes to improved vehicle comfort and handling during normal-inflated operation
while contributing to the tire's runflat durability.
SUMMARY OF THE INVENTION
[0018] The present invention relates to a pneumatic runflat radial ply tire having a tread,
a carcass comprising at least a radial ply, a belt structure located between the tread
and the radial ply, two inextensible beads, and two sidewalls with inserts. The respective
bead regions of the runflat tire are reinforced with circumferentially disposed chippers
in order to minimize the formation of cracks in the chafer region caused by runflat
operations. Each chipper contains reinforcing cords (preferably wires) that are oriented
at an angle of between 30° and 50° in the circumferential direction. The radially
outermost end of each chipper is located between 5% and 20% of the section height
above the wheel rim flange of the wheel-mounted tire. And the chipper is disposed
axially inward of the turnup end of the turned up ply.
[0019] An alternative chipper design is positioned and sized similarly, but comprises two
layers of crossed reinforcement cords.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The structure, operation, and advantages of the invention will become more apparent
upon contemplation of the following description taken in conjunction with the accompanying
drawings, wherein:
FIGURE 1 is a cross-sectional view showing the bead region of the radially inwardmost
portion of the sidewall of a runflat tire;
FIGURE 2 is a finite element analysis diagram showing the locations in the bead region
at which shearing strains were calculated;
FIGURE 2A is a plotted graph showing the FEA-derived strain values at the locations
shown in FIGURE 2;
FIGURE 3 is a side view of the reinforcing cords of a chipper of the present invention,
showing the reinforcing cords and their angles with respect to the circumferential
direction;
FIGURE 3A is a side view of the reinforcing cords of an alternate embodiment of a
chipper of the present invention, showing crossed reinforcing cords and their angles
with respect to the circumferential direction; and
FIGURE 4 is a cross-sectional view of the bead region and radially inwardmost portion
of one sidewall of a runflat tire incorporating the present invention.
DEFINITIONS
[0021] "Apex" or "bead filler apex" means an elastomeric filler located radially above the
bead core and between the plies and the turnup plies.
[0022] "Axial" and "Axially" means the lines or directions that are parallel to the axis
of rotation of the tire.
[0023] "Bead" or "Bead Core" generally means that part of the tire comprising an annular
tensile member of radially inner beads that are associated with holding the tire to
the rim; the beads being wrapped by ply cords and shaped, with or without other reinforcement
elements such as flippers, chippers, apexes or fillers, toe guards and chafers.
[0024] "Carcass" means the tire structure apart from the belt structure, tread, undertread
over the plies, but including the beads.
[0025] "Chipper" refers to a narrow band of fabric or steel cords located in the bead area
whose function is to reinforce the bead area and stabilize the radially inwardmost
part of the sidewall.
[0026] "Circumferential" most often means circular lines or directions extending along the
perimeter of the surface of the annular tread perpendicular to the axial direction;
it can also refer to the direction of the sets of adjacent circular curves whose radii
define the axial curvature of the tread, as viewed in cross section.
[0027] "EMT tire" stands for Extended Mobility Technology and EMT tire means the same as
"runflat tire," which refers to a tire that is designed to provide at least limited
operational service under conditions when the tire has little to no inflation pressure.
[0028] "Equatorial Plane" means the plane perpendicular to the tire's axis of rotation and
passing through the center of its tread; or the plane containing the circumferential
centerline of the tread.
[0029] "Flatspotting" is the tendency of the tread of a tire to sustain a flat spot in the
ground-contacting portion of the tread when a vehicle has been parked or otherwise
sitting for a prolonged period.
[0030] "Gauge" refers generally to a measurement and specifically to thickness.
[0031] "Lateral" means a direction parallel to the axial direction.
[0032] "Ply" means a cord-reinforced layer of rubber-coated radially deployed or otherwise
parallel cords.
[0033] "Radial" and "radially" mean directions radially toward or away from the axis of
rotation of the tire.
[0034] "Radial Ply Structure" means the one or more carcass plies or which at least one
ply has reinforcing cords oriented at an angle of between 65° and 90° with respect
to the equatorial plane of the tire.
[0035] "Radial Ply Tire" means a belted or circumferentially-restricted pneumatic tire in
which at least one ply has cords which extend from bead to bead are laid at cord angles
between 65° and 90° with respect to the equatorial plane of the tire.
[0036] "Sidewall" means that portion of a tire between the tread and the bead.
[0037] "Tread width" means the arc length of the tread surface in the plane includes the
axis of rotation of the tire.
[0038] "Turnup end" means the portion of a carcass ply that turns upward (i.e., radially
outward) from the beads about which the ply is wrapped.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0039] FIGURE 1 is a cross-sectional view of the bead region 10 of a typical two-wedge-insert-per-sidewall
runflat tire. The tire's inner liner 12 lies adjacent to the first wedge insert 14
which is bounded on its far side from the inner liner by the first carcass ply 18.
The second wedge insert 16 is disposed between the first carcass ply 18 and the second
carcass ply 20. The first carcass ply 18 is wrapped around the bead 22 and has a turn-up
end 26 that terminates radially outward from the bead, possibly above the wheel-rim
retainer lip 30 or at the middle section of the sidewall where the section width is
the greatest.
[0040] Shearing strains arise during runflat service in the portion 32 of the bead region
that extends from the bead 22 to the lower part of the tire's wheel-rim retainer lip
30.
[0041] FIGURE 2 is a portion 11 of a finite-element analysis (FEA) diagram of bead region
10 shown in cross-section in FIGURE 1. The wheel-rim retainer lip 30', which is also
known as a rim flange protector, and the bead 22' are denoted. Throughout the specification,
primed numbers represent structural elements which are substantially identical to
structural elements represented by the same unprimed number. The boxed numbers 1 through
8 delineate points at which shearing strains were calculated by means of FEA during
computer-simulated runflat operation of the tire containing a circumferentially disposed
chipper 24 of varying designs in each a tire's two bead regions.
[0042] Since this invention relates to chipper designs for runflat tires, FIGURE 3 provides
a side view (or axially directed view) of the main structural elements of a chipper
40. The chipper 40 is a narrow band 41 of reinforcing cords 42, typically in rubber
and forced into a circular configuration. The angle α of the reinforcing cords 42
of chipper 40 is shown, highlighting a chipper's relationship to the radius R extending
from the tire's axis A. The angle α is measured with respect to the circumferential
direction C
D, which is perpendicular to the radius R about the tire's axis of rotation A.
[0043] The angle α is one variable in chipper design. Another design variable is the chipper's
width, denoted by W in FIGURE 3, which is measured from the radially inwardmost edge
or end 44 of the chipper to the radially outwardmost edge or end 46. The structure
of the cords 42 is yet a third design variable in chipper design. For example, the
cords can be constructed of wire, flexten, rayon, nylon and polyester and typically
with 15-35 EPI (ends per inch). Yet a fourth chipper design variable is the location
of the chipper with respect to the turnup ply 26 shown in FIGURE 1. That is, the chipper's
location can be varied in both the radial and axial directions. In the latter instance,
it can be located axially outward of the turn-up end 26 (FIGURE 1) or axially inward
of the turn-up end as shown in FIGURE 4. Prior art chipper locations place the chipper
axially outward of the turn-up end 26.
[0044] An alternate embodiment of the chipper of this invention is a crossed-cord chipper
43, illustrated in FIGURE 3A as a side view (or axially directed view). The crossed-cord
chipper 43 is a narrow band 45 of reinforcing cords 42, 42', typically in rubber and
forced into a circular configuration. The band 45 consists of two layers 47, 47' of
reinforcing cords 42, 42'. The reinforcing cords 42 of the first layer 47 of the crossed-cord
chipper 43 are at an angle α relative to the circumferential direction C
D, which is perpendicular to the radius R about the tire's axis of rotation A. The
reinforcing cords 42' of the second layer 47' of the crossed-cord chipper 43 are at
an angle α' relative to the circumferential direction C
D, which is perpendicular to the radius R about the tire's axis of rotation A. The
angle α' is preferably equal in magnitude to angle α, but oriented (as illustrated)
in the "opposite direction", i.e., the reinforcing cords 42' are a mirror image of
the reinforcing cords 42 with the radius R being the reflection axis. The first 47
and second 47' layers are suitably bonded together to form the crossed-cord chipper
43 alternate embodiment of the chipper of this invention.
[0045] The angles α and α' are one design variable for the crossed-cord chipper 43. Another
design variable is the chipper's width, denoted by W in FIGURE 3A, which is measured
from the radially inwardmost edge or end 44 of the crossed-cord chipper 43 to the
radially outwardmost edge or end 46. The structure of the cords 42, 42' is yet a third
design variable in chipper design. For example, the cords 42, 42' can be constructed
of wire, flexten, rayon, nylon and polyester and typically with 15-35 EPI. Yet a fourth
chipper design variable is the location of the chipper 43 with respect to the turnup
ply 26 shown in FIGURE 1. That is, the chipper's location can be varied in both the
radial and axial directions. In the latter instance, it can be located axially outward
of the turn-up end 26 (FIGURE 1) or axially inward of the turn-up end as shown in
FIGURE 4.
Finite Element Analysis of Bead Region Shearing Strains
[0046] Referring to FIGURE 2, the boxed locations 1 through 8 show the points at which shearing
stresses were examined by FEA as a function of chipper cord angles α and chipper width
W. All chippers were made of wire cables. The specific test variables or conditions
used in the FEA were as follows, where the angles refer to α (and an equal-valued
α') as defined in FIGURES 3 and 3A:
- Test Condition #1.
- No chipper.
- Test Condition #2.
- 23° wire chipper having standard width ("width" here refers to radial height above
the region adjacent to the bead).
- Test Condition #3.
- 35° wire chipper having a standard width.
- Test Condition #4.
- 23° extended wire chipper has a width of 15 to 25% greater than the standard width.
- Test Condition #5.
- Two crossed-cord chippers have an angle of 35°. The chipper cords (wires) are crossed
symmetrically with respect to the radial direction.
[0047] For each of the above computer-simulated test conditions (TC) the shear strains at
the eight locations designated in FIGURE 2 were calculated by FEA. The results of
the FEA strain calculations are summarized in the following table:
Location shown in FIGURE 2 |
FEA-Derived Strain Values at Test Condition (TC) |
|
TC #1 |
TC #2 |
TC #3 |
TC #4 |
TC #5 |
1 |
3.94E-02 |
3.54E-02 |
2.78E-02 |
2.17E-02 |
1.68E-02 |
2 |
4.50E-02 |
3.42E-02 |
3.08E-02 |
1.94E-02 |
1.53E-02 |
3 |
5.02E-02 |
3.00E-02 |
2.98E-02 |
2.15E-02 |
1.76E-02 |
4 |
5.27E-02 |
2.66E-02 |
2.81E-02 |
2.40E-02 |
2.26E-02 |
5 |
5.41E-02 |
3.16E-02 |
2.74E-02 |
2.91E-02 |
2.65E-02 |
6 |
5.08E-02 |
2.93E-02 |
2.42E-02 |
3.23E-02 |
3.15E-02 |
7 |
3.77E-02 |
2.26E-02 |
1.98E-02 |
2.50E-02 |
2.41E-02 |
8 |
2.96E-02 |
1.95E-02 |
1.63E-02 |
2.04E-02 |
2.07E-02 |
[0048] The FEA-derived numbers listed in the table above are depicted graphically in FIGURE
2A, showing the bead regions strains at the eight locations defined in FIGURE 2.
[0049] FIGURE 2A indicates that by increasing the cord chipper angle from the standard value
of about 23° to a value in the range of 30° to 50°, the stiffness in the bead area
will be better balanced in terms of the maximum magnitudes of the shearing stresses.
Or, in other words, for the six locations 1 through 8 shown in FIGURE 2A, the relative
strain differences, or cycle amplitude, at each station point 1 through 8 and for
each of the five Test Conditions #1 through #8, are minimum for the Test Condition
#5, i.e., 35° crossed-cord chipper.
[0050] The maximum magnitudes of the shearing strains were calculated as a comparison of
the shearing strains that occurred in the eight locations of that portion of the bead
region most adjacent to the ground-contacting portions, i.e., the footprint area,
where the strains are maximum, of the tire and the shearing strains in the corresponding
eight locations on the part of the bead region farthest removed from the ground-contacting
portion of the tire where the strains are minimum. In other words, the shearing strains
were compared between the bottom of the tire (i.e., where the tread makes contact
with the ground) and the top tire (which is unloaded). The difference between the
shear strains at the bottom of the tire and the top of the tire (in relation to the
ground-contacting portion of the tire) is called the shear strain cycle amplitude.
The effect of the reduction of the shear strain cycle amplitude is to reduce or eliminate
the cracking in the chafer area of a tire operating in the runflat mode. This increases
the runflat operational distance.
[0051] An improved balance between top and bottom of the tire, that is, a minimum shear
strain cycle amplitude, reduces the amplitude of the standing waves which are known
to provoke the high speed failure in normal-inflated operation. Furthermore, the increase
in chipper cord angle α from 23° to a value in the range of 30° to 50° beneficially
influences the stiffness/damping characteristics of the tire. More specifically, the
increase in the angle α leads to a better stiffness/damping balance between the bead-region
portion 32 (in FIGURE 1) of the tire and the corresponding tread portion. As a consequence
of the improved stiffness/damping balance, a runflat tire containing such high-angle
chipper provides a more comfortable, softer ride as well as improved flatspotting
performance during normal-inflated operation.
[0052] The FEA also showed that the width W of the chipper 24 (FIGURE 3) works well as a
useful design parameter for tuning handling/steering performance of runflat tires.
However, it was determined that in order to optimize the effect on riding comfort,
as well as high-speed inflated durability and flatspotting, the width W should be
kept to a minimum. Referring to FIGURE 4, the optimum width W was determined to be
such that the radially outermost portion 52 of the chipper 54 incorporated in a typical
runflat tire 49 should be located at a distance D of 5% to 20% of the section height
SH of a runflat tire 49 the distance from the axially inwardmost point of the tire
in the head area to the axially outwardmost point of the tire on the tread at the
equatorial plane.
[0053] In summary, the finite element analysis showed that the higher angle α of 30° to
50° significantly reduced the shear strain cycle amplitude. The use of a crossed-cord
chipper (the crossed cords each having an angle of 30° to 50°) was also beneficial
in reducing the shear strain cycle amplitude by around 40%. If the angle of the chipper
were greater than 50°, the cords within the chipper would have a tendency to cut through
the ply and/or sidewalls during normal or runflat operations.
Balanced Chipper Design Usage in Runflat Tires
[0054] In runflat or EMT tires, the use of chippers contributes to the tire's runflat operational
service life. On the other hand, the increased rigidity associated with the use of
chippers tends adversely to affect the ride and comfort because of the stiffening
effect of the chippers in combination with stiffening effects of the EMT tire's sidewall
wedge-insert reinforcements.
[0055] A balanced design for a chipper-reinforced bead assembly of a tire would include
stress characteristics that lead to reduced flexural energy generation (and heat buildup)
and to strain characteristics that can be uniformly borne by mutually adjacent tire
components in the bead region. The objective of a balanced design is to achieve high-speed
handling and steering benefits without compromising riding comfort due to the increased
rigidity associated with typical chipper designs.
[0056] Such a balanced design concept incorporating the benefits of chippers could apply
to EMT tires by reducing the chafer-region flexure that leads to shear-stress-induced
cracking and to a compromised runflat service life. In EMT tires, optimum chipper
design would provide benefits to high-speed handling and steering as well as to runflat
operational life. The latter increase in runflat operation life would be brought about
by a chipper design that can reduce shearing strains in the bead-region portions of
the sidewalls during runflat operation while, at the same time, not simultaneously
increasing the normal-inflated overall stiffness of the sidewalls.
Actual Field Test
[0057] A chipper having the larger angle α of 35° was tested on a number of otherwise original
equipment type tires. Compared to a construction with a chipper having a standard
angle of 20-25°, high-speed reliability during inflated operation and general riding
comfort were significantly improved without adversely affecting other performance
variables. During the non-inflated operating mode, the cracking in the chafer area
was substantially eliminated.
[0058] Shown in FIGURE 4 is chipper construction of the present invention incorporated on
a tire 49 mounted on a wheel rim 55. The tire 49 is mounted on wheel rim 55 so that
the bead region 61 which includes bead 62 is seated against the wheel flange 56 and
extends from the bead 62 to the lower part of the tire's wheel-rim retainer lip 63.
The chipper 54 is located axially inward of the ply turnup end 58 of ply 57. In that
location, the chipper 54 extends between the radially outwardmost location 58 corresponding
to the end 58 and the location 60 that is close to the bead 62, i.e. 1-10 mm (millimeters)
above the bead. If the distance were greater than 10 mm, the chipper would no longer
provide adequate support to reduce the occurrence of cracking in the chafer region.
The chipper 54 in this location that is axially and radially inward, with respect
to the axis of rotation of the tire, of the turnup end 58 and having cord angles α
of between 30° and 50° and a width W that is adequate to allow the chipper's radially
outwardmost end 50 to be at a distance D above the lower part of the tire's wheel-rim
retainer lip 63 of between 5 and 20% of the section height of the tire. The chipper
50 can incorporate either the parallel cord or the crossed cord design shown in Figures
3 and 3A, respectively. The chipper 50 is contributory to minimizing the shear strain
cycle amplitude, which leads to a reduction of cracking in the chafer during normal-inflated
and runflat operation. The improved chipper designs of the present invention also
contributes to low-speed comfort during inflated operation, and to handling during
both normal-inflated and runflat conditions.